The present invention relates to flow control of fluids by a diaphragm valve.
More particularly, the invention provides a pressure controlled, diaphragm operated valve, and a diaphragm for use in said valve having an extended operating life. The present invention is however not concerned with valves of the type where the diaphragm is used directly in contact with and to control flow of the fluid being controlled, such valves being limited to pressures of approximately 4 kg/sq cm or less.
The primary advantage of diaphragm operated valves relative to spool operated valves lies in their prevention of any leakage of the fluid being handled, which leakage can occur in other valves through the spool gland. The diaphragm which moves the valve stem can itself be operated by mechanical, electrical or fluid pressure means, the latter method being particularly advantageous as adequate valve operation forces even for large valves can easily be applied, and the pressurized liquid or gas passing through the valve can itself be utilized for this purpose, thus eliminating the necessity of providing some outside energy source. Where some external power source is used for operating the diaphragm a different advantage is achieved--the diaphragm is isolated from the fluid being handled, which valve can then be used for fluids of a type which would chemically attack the diaphragm were it to be in contact therewith.
Diaphragms are commonly made of natural or synthetic rubber, and may be textile reinforced, and teflon coated. Diaphragms are subject to flexing during operation, and have a finite operating life eventually requiring their replacement, which is preferably carried out before failure as part of a preventive maintenance program. An extended operating life of the diaphragm is of course highly desirable, and is essential in valves which are frequently operated.
Applicants market a valve described in this company's catalogue "Raphael, diaphragm actuated valves series--V". which comprises a body, cover, a control unit with a compression spring, a shut-off disk and a valve status indicator. This indicator is rigidly attached to the valve stem by a nut with slots.
The valve is operated by directing the fluid being handled to either side of the diaphragm.
The control unit includes lower and upper covers having an elastic diaphragm disposed therebetween. Metal disks are mounted on both sides of the diaphragm, the upper disk acting as a seating for said spring which operates the diaphragm under low or zero pressure. The diaphragm and both its disks are mounted on the stem also carrying the shut-off disk. The diaphragm of this valve is designed in accordance with US Pat. No. 3,762,681 to McKinney et al, and is reinforced with cotton or nylon.
A disadvantage of the described valve is that under conditions of low pressure differential and low flow rates, axial stem vibrations occur as a result of instability between the fluid pressure and the force exerted by the compression spring. Such vibrations are damaging to the flexible diaphragm and shorten its operating life.
Similar vibrations occur when the shut-off disk is 5-10 mm from closure. Such vibrations generate unwanted noise as well as causing the premature destruction of the diaphragm.
A further disadvantage resulting from the use of said compression spring is that its resistance must be overcome by the fluid being passed by the valve, resulting in undesirable pressure drop when fluid flows through the open or partially open valve.
The status indicator of the valve is attached to the valve stem in a rigid manner which causes alignment problems with regard to the sleeve bearing and seals through which it projects.
The BERMAD CO., in their catalogue "BERMAD, Control Valves, Model 700" describe a valve of a generally similar type, except that the compression spring is disposed between the shut-off disk and the diaphragm. The BERMAD valve suffers from the same disadvantages described above. Additionally, should the compression spring fail and need replacement, the pipeline must be shut off for dismantling of the valve during repair.